Breast carcinoma is the second leading cause of female cancer death in the US. It is estimated that in 2001 alone more than 190,000 women were diagnosed with invasive breast cancer, with more than 40,000 deaths. The major fatal complication associated with breast cancer is metastasis. Whereas original tumors are often curable, at present there is no efficient curative therapy to treat advanced metastatic breast disease. After escaping the primary tumor, breast cancer cells frequently colonize bone and bone marrow. In more than half of the patients with advanced breast cancer, the disease has metastasized to bone, causing intractable pain, pathological fracture, and spinal cord compression (1). Unraveling the molecular and cellular mechanisms of breast cancer bone metastasis is critically important for developing of new efficient therapies of this devastating disease. Metastatic bone disease is a unique event on many levels, involving multiple cell-cell and cell-extracellular matrix interactions. It was recently suggested that specific tumor cell adhesive interactions with the microvascular endothelium of a target organ could be an important factor in defining tissue and organ specificity of cancer metastasis (2, 3), including breast cancer bone metastasis (3). There is also a fast-growing body of experimental evidence indicating that crosstalk between neoplastic cells and the bone microenvironment, including its cellular components and extracellular matrix (ECM), plays a critical role in determining the fate of metastatic tumor cells and the outcome of the metastatic process. Therefore, there is an urgent need to develop experimental models in which metastatic cancer cells interact with human bone marrow microvascular endothelium in the environment of active human bone marrow and human bone marrow ECM. Only such models can provide accurate, adequate, and reliable information on the molecular and cellular mechanisms involved in the metastasis of human breast cancer to bone. However, there is an apparent lack of such in vivo experimental systems. The major limitation of the animal models currently used for studying breast cancer metastasis is that they employ human cancer cells interacting with rodent microvasculature and stromal components. In addition, they do not allow for uninterrupted observation and imaging of metastasis-associated cellular and molecular events. In this application, we suggest an integrated approach, combining a unique animal experimentation technique for generating adult human spongeous bone (AHSB) xenografts implanted into dorsal skinfold observation chambers in immunodeficient (NOD/SCID) mice, fluorescent tagging of important molecular structures in metastatic breast carcinoma cells such as focal adhesions and actin stress fibers, and intravital epifluorescent and laser scanning confocal microscopy for real-time in vivo imaging of metastatic cancer cell adhesion, extravasation, clonogenic growth, and neovascularization, as well as in vivo imaging of intracellular metastasis associated molecular events. The great advantage of this experimental system will be that breast cancer bone metastasis will be studied in a the context of a genuine human spongeous bone microenvironment, while intravital epifluorescent and laser scanning confocal microscopy will allow for optical imaging of important cellular and molecular interactions.